But in the worst-case scenario, the Deorbit Vehicle won’t be ready. Without it, a protocol agreed on in 2024, which relies on the Russian Progress spacecraft, could come into play. There will be vexing trade-offs to consider. Allowing the ISS to descend toward Earth on its own saves gas, which the ISS will need when it’s finally time to eject into the atmosphere and ensure a burial at sea. But a slow descent jeopardizes the machinery needed to maintain remote control.
A controlled deorbit requires the use of several core systems, including those for communications, power, and avionics. Some of the ISS machinery was not specifically certified to perform in a depressurized environment. (NASA believes that critical systems would remain operable, based on technical analyses, and emphasizes that many of these systems are already used in vacuum.) Another thing to worry about: the ISS losing control over its orientation in space. The spacecraft could start tumbling, flipping the station’s solar arrays away from the sun, taking the primary source of power with it.
And no matter what, the plan to use Russian assets remains problematic, since the ISS would have a “shallower reentry,” NASA says, and sprinkle surviving debris over a larger-than-desired area. Still, NASA would retain significant control over where any of these extant shards might plop down. They’ll probably land in the ocean, just as the space agency has always hoped. Sure, the station would have died before its time, but the thing was getting old. Most likely, it will be fine.
But what if it’s not fine? Even back in 1996, before a single component of the ISS was launched into orbit, NASA foresaw the possibility of an even worse worst-case scenario: an uncontrolled reentry. The crux of this scenario involves multiple systems failing in an improbable but not completely impossible cascade. Cabin depressurization could damage the avionics. The electrical power system could go offline, along with thermal control and data handling. Without these, systems controlling coolant and even propellant could break down. Unmoored, the ISS would edge slowly toward Earth, maybe over a year or two, with no way to control where it is headed or where its debris might land. And no, we could not save ourselves by blowing the station up. This would be extremely dangerous and almost certainly create an enormous amount of space trash—which is how we got into this hypothetical mess in the first place.
The atmosphere is a ruthless incinerator, and, no matter how the ISS comes down, most of it would be vaporized. But there’s still that chunk of station that could survive reentry. In the best case, where we’re prepared, air traffic controllers and maritime authorities can issue alerts. The station will shed pieces into the sky, and Australians might get a nice view before things kerplunk into the sea. Then the remains of this historic feat of human engineering will sink to the ocean floor, another carcass left to the algae and the microplastics.